The rate at which heat is lost through a building element is called its U value, measured in W/m2K, which means the rate of heat energy (W) lost per 1m2 of the product for every oC (or degree Kelvin) difference between the outside temperature and the inside one. The lower the U value the better, as it demonstrates more resistance.
If you have 100m2 of a wall with a U value of 0.25, and it is 20oC inside, but just freezing at 0oC outside, the rate of heat lost will be: 100m2 x 0.25 W/m2K x 20oC = 500W = 0.5kW
If these conditions stayed the same for 24 hours, then 12 kWh of heat energy would be lost through the walls alone. This would be a highly insulated wall. Heat moves from warm places to cold ones, so you want to think about putting insulation on the outside of a warm area, to keep the warmth in.
There is good and bad ventilation: draughts are the main constituent of the bad category. The real difficulty is getting good advice on what is the essential minimum of good air circulation.
Some air movement from inside to outside the house is needed to dilute ordinary living and cooking smells and bathroom moisture, and from outside to inside to provide for gas-fired appliances and boilers. The rate of movement is usually measured in air-changes-per-hour (ach). At 1ach, the warm air in the room is replaced every hour and the new air needs heating. This is a major component of our heating bills, so brand new homes are meant to have a maximum level of air flow and a high level of air tightness. It is one of the reasons why new homes should be more energy efficient.
There is considerable uncertainty and mythology about air movement, particularly as it is impossible to measure in an ordinary house without using pressure-testing. It is essential for human health that any harmful emissions from gas-fired appliances, particularly carbon monoxide, are removed. Hence, more boilers have balanced-flues so that the air intake and the exhausted air never get into the home. With individual gas appliances, such as gas hobs, there should always be an extractor hood above them that is used whenever the hob is lit and that takes the air outside.
Old homes have so many worn parts that normally they are too leaky. This is particularly true of doors and windows, but also the mortar in the brickwork can become cracked and no longer air tight. In all homes, any inserted pipes or wires, to the outside, into the loft etc., can also result in unnecessary air movement. Sealing these up properly is a useful start. So, don’t just think of draughts as uncomfortable, they are also an indicator of unnecessarily high heating bills.
Some air movement takes place within the house, mixing the air from hot rooms with that from cold rooms. This may create an uncomfortable draught, particularly where there is a considerable temperature gradient within the home. Think of it just as air mixing – it is not really worrying.
In extreme cases, poor ventilation combined with cold, uninsulated surfaces can result in condensation and mould. In many homes, this is most likely to be found in rooms that are rarely heated, or behind furniture, where there is little air movement, or on cold windows, often with metal frames. In extreme cases, the result is unpleasant smells, unsightly mould and health risks (for instance asthma). It is important to BOTH reduce the source of the moisture combined with greater air movement and to limit the number of cold surfaces on which the condensation can form. In a warm, energy-efficient home with good insulation there is rarely any condensation.
Which have produced a useful guide to identifying and dealing with damp.